A single axis levitator often includes a pair of facing walls spaced along the axis of the levitator, with one wall generally being flat and vibrated by a transducer and the other wall being a concave reflector. An object tends to be levitated along the axis, at locations spaced from the concave reflector that are odd multiples of a quarter wavelength of the acoustic energy. The interference between the incoming plane wave approaching the reflector and the curved wave reflected therefrom results in an acoustic well or levitation location centered on the axis. Additional ring-shaped levitation locations are produced about the on-axis location, but are relatively weak, and the on-axis levitation location is generally of greatest importance. The levitation force is strongest along the levitation axis and is typically about one-tenth as great in a perpendicular direction. Thus, in a one-G gravity environment, which exists on the Earth's surface, the levitation axis is usually vertical.
In an ideal single axis levitation system which is symmetric about the levitation axis, there is no angular torque tending to orient the sample about the axis. However, in actual single axis levitators, there is generally sufficient assymmetry to produce small angular torques, which can initially orient the sample at low sound field levels. However, when more intense sound fields are established, they generate fluid flows such as turbulence or acoustic streaming, which can produce torques that rotate the sample. A similar situation exists in single mode resonant levitators of cylindrical, spherical and rectangular types wherein the levitating mode results in axial symmetry. It would be desirable to controllably produce orienting torques that could counteract such fluid-flow caused torques to orient or rotate the object.